1. Field of the Invention
This invention relates to a software process which creates or increases the sense of field depth of an existing two-dimensional image by defocusing the image.
2. Description of the Related Art
Producing a realistic two-dimensional image has been one of the important fields of image processing. An image which is in good focus is an important part of reality, but having the image in good focus is not enough. When looking at a two-dimensional image, an object which was originally a three-dimensional object often looks realistic because surrounding objects are blurred or out of focus, creating a sense of field depth. Unrealistic images can occur in two situations. First, unrealistic images occur when a computer generates images through three-dimensional rendering, and there is no information on camera optics available. Without camera optics, objects cannot be defocused and realistic images cannot be constructed. A second situation in which unrealistic images occur is when the focal depth of the camera is very deep, as is often seen with recent digital cameras because the optics of these cameras have very short focal length. All areas of an image produced by digital cameras are often in focus, resulting in very poor sense of field depth.
Light ray tracing is the straight approach to defocus images for computer generated three-dimensional objects. There have been several such works or inventions, including U.S. Pat. No. 5,986,659 to Gallery et al., U.S. Pat. No. 6,028,606 to Kolb et al., and U.S. Pat. No. 4,731,864 to Modla. These are based on a light-ray tracing process that simulates light rays emitted from an object and passing through a camera lens, aperture (sometimes called stop or iris), shutter, and films. In such a ray tracing process, as well as in actual optics, if a very small spot such as a star becomes defocused, it just looks like a disk over which the original amount of light is uniformly distributed. The shape of the disk is identical to the shape of a camera aperture. The original spot is defocused to a disk with its edge sharply defined. This disk may be called a defocused disk. The size of the disk depends on the distance between an emitted point and the camera optics. If the image is more focused, the size of the disk becomes smaller. A defocused image is a summation of the defocused disks from all points or pixels. Often, the defocused disks from bright points remain seen as disks after the summation, giving a good sense of field depth. These simulations reproduce such defocused disks, although their processes are very detailed and precise, therefore costing a significant amount of computing time. Such disks are valuable to study or design a camera or an optical system, but extravagant in providing a good sense of field depth to the human eye.
For images taken with a digital camera or images taken with deep focal depth, people sometimes try to use existing software products to improve the sense of field depth. None of them, however, reproduce the defocused disks essential for sensing field depth, as mentioned above. In these products, such as Adobe System Inc.'s Photoshop or Jasc Software Inc.'s Paint Shop Pro, defocusing images is often misunderstood as Gaussian blurring or other types of blurring. Gaussian blurring distributes light according to Gaussian distribution and does not produce the defocused disks.
To make matters worse, these products apply their numerical operation on a pixel value itself, not on the amount of light. This is a very serious misunderstanding in this field. The value stored in a pixel of an image corresponds to the density of photographic films or to the sensitivity of human eyes, both of which are the amount of light not in linear scale, but in logarithmic scale. Actual physical processes, on the contrary, require operation on the amount of light in linear scale. One photon plus one photon is two photons and one unit of light-energy plus one unit of light-energy is two units of light-energy. Thus, if the scale is logarithmic, the result is indeed invalid. Since the defocusing-operation of the straight approach is applied directly to a pixel value, the results are invalidly and unrealistically produced defocused images. The film or human eyes have a limited dynamic range for a large amount of light. These situations become most apparent when the characteristics of films or human eyes are correctly handled. If the amount of light is very large, saturation in sensitivity occurs, and the light appears just white in films or eyes. Consider-a datum in a pixel for bright light, for example, such as 253 or 254 in eight-bit range (0 to 255). The difference of just one digit in this brightest range may amount to ten times or more difference in the amount of light (energy or number of photons) in the real world. If a software process does not account for the real amount of light and creates a defocused image, the effect is completely underestimated, resulting in a very unrealistic look. In such a case, a bright spot in an image spreads only a slightly brighter blur to surrounding pixels. A bright point in a realistic image should spread as a sparkling defocused disk to surrounding pixels as actually seen in real photography.
All the software processes heretofore known suffer from one or more of the following disadvantages:
(1) Light-ray tracing software costs computing time.
(2) Existing image-processing products do not produce correct defocused images. They cannot produce the defocused disks which are the essential part of defocused images as actually seen in the real world.
(3) Existing image-processing products do not treat the amount of light correctly. The summation of amount of light should be taken in linear space, not in logarithmic scale.